Serious health impairments can be triggered by an elevated intraocular pressure. In particular, the optic nerve can be damaged by the elevated intraocular pressure which causes so-called glaucoma with restrictions of the visual field.
Three basic principles are known for checking the intraocular pressure, namely impression tonometry, applanation tonometry and noncontact tonometry. The impression tonometer measures the depth of the indentability of the cornea caused by a metal stamp loaded with a known weight. For the same weight the indentability is inversely proportional to the intraocular pressure, that is the indentability is greater, the lower is the intraocular pressure and conversely. A disadvantage with impression tonometry is that the placement of the tonometer and the impression of the metal stamp additionally increase the intraocular pressure so that the measured pressure does not correspond exactly to the actual intraocular pressure. Furthermore, the placement of the stamp on the cornea of the patient's eye is relatively stressful for the patients.
Furthermore, so-called applanation tonometers are also known for measuring the intraocular pressure, its measurement being based on application of the applanation principle. The applanation principle starts from Ingbert's law which states that the pressure in a spherical container filled with liquid corresponds to the counterpressure which flattens a certain surface of this sphere. The intraocular pressure can be measured on the basis of this law in two different ways. According to a first alternative, a tonometer with constant weight can be used and the flattened surface can be measured. According to an alternative method of measurement, the force required to flatten a known surface of constant size is used A Perkins applanation tonometer is known, which consists of a plastic cylinder whose lower planar end is provided with a gradation. A magnifying glass is located at the upper end. After instilling a fluorescent liquid into the conjunctival sac, the diameter of the applanated corneal surface can be determined by optical reading off on the gradation scale. In this case, the intraocular pressure is determined by means of a constant force.
In addition, an applanation tonometer operating on the principle of an applanated surface of constant size is known. In this case, the cornea is flattened using the quadrilateral base of a glass prism. The intraocular pressure is measured by intensifying the pressure of the prism on the eye until the flattened circular region of the cornea is at the same level as the four sides of the prism base. A disadvantage with applanation tonometers again is that as a result of the deformation of the cornea by means of an actuating element, considerable stress is produced for the patients.
So-called noncontact tonometers were developed to avoid this stressing produced by contact with a deforming tool. In these noncontact tonometers actuating devices are provided for deforming the cornea with which the cornea is deformed free from contact. For this purpose, a puff of compressed air is produced for example and directed onto the cornea. In known noncontact tonometers air puffs are directed onto the eye in the direction of the optic axis whereby the cornea is increasingly flattened and finally indented. To measure the deformation of the cornea, an obliquely incident bundle of parallel light rays is directed onto the cornea and the light reflected by the cornea is measured as a measurement signal. For this purpose, the reflected light can be intercepted by a light sensor, for example, where the light intensity measured by the light sensor varies as a function of the applanation of the cornea caused by the air flow.
A disadvantage in all known methods of measurement is that when measuring the intraocular pressure, the counterpressure caused by the elastic deformation of the cornea is not taken into account. This is because the cornea itself is stretched over the vitreous body in the fashion of an elastic membrane so that during the measurement of the intraocular pressure a certain amount of force is required for its deformation which is included in the measurement results in a falsifying manner. This falsification is of a different magnitude in different patients since the properties of the cornea, especially its thickness and elasticity, vary within certain limits.